A heat gun and a plastic scraper/putty knife for most. For the ones that are completely cracked and brittle, this may not work. For that I use a 3M™ Stripe Off Wheel. You can read more about those in this thread...

The heat gun and plastic scraper will work great... but this is one case where a hair dryer may be the better choice, and even then careful with the heat.
Just enough to soften the vinyl and the glue being careful not to start to soften the gelcoat
Scrubbing the cold residue with ordinary hand cleaner, softscrub, or a heavy solution of carwash soap and scotchbrite.
Follow this with a good coat of wax and gelgloss.
The results will likely be very nice until the sun shines JUUSST right on it and you can still see faint evidence of the decal, because the decal had protected the gelcoat under it for the last 24 years.

A heat gun and a plastic scraper/putty knife for most. For the ones that are completely cracked and brittle, this may not work. For that I use a 3Mô Stripe Off Wheel. You can read more about those in this thread...

I use a hair dryer believing the heat gun to be too hot. To get the old stubborn glue, goo-gone or WD-40 works great. I've used acetone on occasion for the last really stubborn stuff but always sparingly and then rinse any residue off. Wear chemical proof gloves and consider a proper chem mask. Nasty stuff! This worked great for removing all the old decals and the lateral stripe from our old Trillium. How to handle the decal "shadow"? New decals, new stripe! Oh and I finally applied Zep this year - looks brand new!

That shiny stuff after the decals are removed, is just telling you how aged your trailer is. Scuff it up, apply new decals.. wax or ZEP... then go camping! It's NOT about how "new and fancy" your trailer is... it's about the MEMORIES!

+1 on just put new decals over old decal shadows. And the 3M stripe off wheel worked fantastic for me. Thing is sold in auto paint shops for body shops. It's essentially an eraser on a shaft. Run it in a variable speed drill. Cleans the decals off really fast.

Would caution you to make sure trailer surface is clean and not get dirt on the wheel. Dirt on trailer or stripe off wheel will leave scratches.

+1 on just put new decals over old decal shadows. And the 3M stripe off wheel worked fantastic for me. Thing is sold in auto paint shops for body shops. It's essentially an eraser on a shaft. Run it in a variable speed drill. Cleans the decals off really fast.

Would caution you to make sure trailer surface is clean and not get dirt on the wheel. Dirt on trailer or stripe off wheel will leave scratches.

Perfect Roger. I've read, over the years, folks that have removed decade-old decals quickly with the "eraser." I'm just grateful the ones on my 26 y/o Scamp are in good shape!